1. Technical Field
Generally, the invention relates to a sieving device. Particularly, the inventions relates to a sieving device having a screen and a screen frame extending around a periphery of the screen which uses an ultrasonic drive to vibrate the screen at high frequencies. Specifically, the invention relates to a sieving device in which an ultrasonic probe rests on the screen frame to vibrate the screen at a periphery thereof.
2. Background Information
Traditional sieving devices typically include a base, a frame resiliently mounted on the base and a screen or screens extending across the frame. A low frequency vibratory drive in the speed range of 8 Hz to 30 Hz is mounted to the frame and includes eccentric weights. Specific vibratory motions are established in the frame by the low frequency vibratory drive generating screen accelerations up to a range of 7 g.
These prior sieving devices have been used for screening fine materials and powders. Stainless steel woven mesh screens are usually stretched tightly and attached to the screen frame. The vibration of these sieving devices typically enhances gravity separation of particles presented to the screen. Where fine particles are to be sieved, the vibration also has a negative effect in that the fine particles become suspended above a boundary layer formed above the vibrating screen.
These fine particles can be sieved through the mesh screens by vibrating the screen or screen frame relative to the base at high frequencies using an ultrasonic drive or device. These ultrasonic devices include an electro-mechanical transducer, the body of which is rigidly mounted with respect to the frame and is coupled to the screen by a metal probe. The low frequency vibration of the screen effected by the oscillation of the frame serves for bulk movement of the material on the screen allowing all layers of material to be presented to the screen, while the high frequency vibration of the screen effected by the ultrasonic device serves to prevent blinding of the apertures of the screen otherwise caused by material adhering to the screen or by particles of the material locking together to bridge the apertures of the mesh screen. These ultrasonic vibrating probes are typically mounted to separator frames with a direct mechanical attachment. The separator frames are directly attached to the screens at the centers thereof. Alternatively, ultrasonic devices have been bonded directly to the screen.
One problem with these prior art sieving devices is that the fine mesh screens tend to weaken in structure as the ultrasonic vibration is applied directly thereto. Also, with the vibrational probe attached directly to a center of the screen, the ultrasonic vibrations tend to dissipate and attenuate after traveling only a short distance from the center attachment and fail to adequately vibrate the peripheral outer portion of the mesh screen. As the vibrations travel from the center of the screen outwardly, the radial surface area of the screen increases dispersing and attenuating the vibrational waves. Further, sieving devices having the ultrasonic probe attached to a center of the screen must have supports and wires extending from the periphery to the center of the screen to drive the probe. The material being sieved through the mesh screen accumulates on these supports and wires after it passes through the screen. This accumulation of material must be cleaned from the supports and wires and often is not salvageable for its intended use.
One patent which attempts to address this problem of dissipation and attenuation of vibrational energy caused by the center attachment is U.S. Pat. No. 5,542,548 which discloses a screening system with a resiliently mounted frame having a screen extending there across. The ultrasonic probe of this prior art sieving device is rigidly attached to a separator frame in one embodiment and directly and rigidly attached to the screen frame in a second embodiment.
Although this prior art sieving device is adequate for the purpose for which it is intended, the rigid attachment between the ultrasonic probe and the sieving frame or screen frame may be difficult to replace in the event that the ultrasonic probe breaks or malfunctions. Additionally, the sieving frame or screen frame must be specially manufactured to include the rigid attachment of the ultrasonic probe thereto or the ultrasonic probe must be welded or otherwise secured to the sieving frame or screen frame.
Therefore, the need exists for a sieving device which vibrates the screen frame around the periphery of the mesh screen at high frequencies using an ultrasonic device, in which the probe of the ultrasonic device rests or lays on the screen frame free of rigid attachment thereto and which is easily retrofit to existing sieves.